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Technical Report NREL/TP-550-46916 December 2009
Preliminary Assessment of the Energy-Saving Potential of Electrochromic Windows in Residential Buildings David R. Roberts
National Renewable Energy Laboratory 1617 Cole Boulevard, Golden, Colorado 80401-3393 303-275-3000 • www.nrel.gov
NREL is a national laboratory of the U.S. Department of Energy Office of Energy Efficiency and Renewable Energy Operated by the Alliance for Sustainable Energy, LLC
Contract No. DE-AC36-08-GO28308
Technical Report NREL/TP-550-46916 December 2009
Preliminary Assessment of the Energy-Saving Potential of Electrochromic Windows in Residential Buildings David R. Roberts
Prepared under Task No. BET98001
NOTICE
This report was prepared as an account of work sponsored by an agency of the United States government. Neither the United States government nor any agency thereof, nor any of their employees, makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise does not necessarily constitute or imply its endorsement, recommendation, or favoring by the United States government or any agency thereof. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States government or any agency thereof.
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iii
Executive Summary Electrochromic (EC) windows provide variable tinting that can help control glare and solar heat gain. We used BEopt software to evaluate the performance of EC windows in prototypical energy models of a single-family home in Atlanta. The windows were assumed to operate automatically, tinting in response to incident solar during the cooling season. The models predict the EC windows will produce whole-house source energy savings of 9.1% and whole-house electricity demand savings of 13.5% in a 2006 IECC-compliant home, and source energy savings of 3.2% and demand savings of 10.3% in a 50%-level Building America home.
iv
Contents Executive Summary ........................................................................................................... iii
Introduction ......................................................................................................................... 1
Approach ............................................................................................................................. 1
Prototype Model Definitions........................................................................................... 1
DOE-2 Modeling Capabilities ........................................................................................ 2
Impact of Seasonal Control ............................................................................................. 4
Response to Controls ...................................................................................................... 5
Results ................................................................................................................................. 6
Savings in a 2006 IECC-Compliant Home ..................................................................... 6
Savings in a 50% Building America Home .................................................................... 8
Cost-Competitive EC Windows .................................................................................... 10
Conclusion ........................................................................................................................ 11
Future Work ...................................................................................................................... 11
References ......................................................................................................................... 12
v
Figures Figure 1. EC window tinting response to total solar incident on window ....................... 3
Figure 2. Heating and cooling source energy use in Atlanta reference home .................. 4
Figure 3. EC window tinting for each hour of the year—west-facing window ............... 5
Figure 4. EC window tinting for each hour of the day ..................................................... 6
Figure 5. Fully tinted equivalent hours for each hour, by orientation .............................. 6
Figure 6. BEopt output for source energy savings for EC windows relative to low-e, low-SHGC windows ......................................................................................... 7
Figure 7. Electricity demand for cooling with EC windows and code-compliant reference windows in 2006 IECC home on July 2 and 3 .................................. 8
Figure 8. BEopt output for source energy savings from EC windows in 50% Building America home ................................................................................................... 9
Figure 9. Electricity demand for cooling (compressor and fan) with EC windows and reference windows in 2006 IECC home on July 2 and 3 ................................ 10
Figure 10. BEopt output. EC glazing is competitive at approximately $20/ft2. ............... 11
Tables Table 1. General Characteristics of the Prototype Energy Models ................................. 1
Table 2. Prototype Model Performance Levels .............................................................. 2
Table 3. Characteristics of SageGlass EC Glazing and DOE-2.2 Window Library Entry .................................................................................................................. 3
Table 4. Source Energy Impacts of Disabling EC Control During Heating Season (MMBtu/yr)....................................................................................................... 4
Table 5. Energy Cost Impacts of Disabling EC Control During Heating Season ($/yr)........ 5
Table 6. Source Energy Savings From EC Windows in 2006 IECC home (MMBtu/yr) ... 7
Table 7. Energy Cost Savings From EC Windows in 2006 IECC Home ($/yr) ............. 7
Table 8. July 3 Peak Electricity Demand Savings From EC Windows in 2006 IECC-Compliant Home (kW) ..................................................................................... 8
Table 9. Source Energy Savings From EC Windows in 50% Building America Home (MMBtu/yr)....................................................................................................... 9
Table 10. Energy Cost Savings From EC Windows in 50% Building America Home ($/yr) ................................................................................................................. 9
Table 11. July 3 Peak Electricity Demand Savings From EC Windows in 50% Building America Home (kW) ....................................................................................... 10
1
Introduction Electrochromic (EC) windows provide variable tinting that can help control glare and solar heat gain. The windows darken in response to changes in applied electrical voltage1
This report summarizes the results of a simple energy modeling exercise conducted to assess the impact EC windows might have in residential applications.
and can be controlled manually or automatically. EC windows are available commercially, though currently their use is generally limited to commercial building applications and residential skylights. A study by Lee et al. (2006) that focused on commercial building applications reported significant energy and demand savings, based on empirical testing and energy modeling.
Approach
Prototype Model Definitions We used energy modeling to assess the potential impact of installing EC windows in a single-family home and used the BEopt2
Table 1
software to develop and analyze prototype single-family home models. General characteristics of the prototype models are shown in
. Table 1. General Characteristics of the Prototype Energy Models
Conditioned floor area 2,500 ft2 Conditioned volume 22,500 ft3 Number of stories 2 Number of bedrooms 3 Gross above-grade wall area 2,574 ft2 Window area 450 ft2 (18% of conditioned floor area) Window orientation 50% west, 25% east, 12.5% north, 12.5% south Window interior shading factors 0.85 winter, 0.7 summer* Door area 40 ft2 Set point temperatures 71 heating, 76 cooling Mechanical ventilation Exhaust only, ASHRAE 62.2 levels Space/water heating fuel Natural gas Internal gains Building America Benchmark (Hendron 2008) Lighting/appliance/plug schedules Building America Benchmark
* This is the standard assumption in Building America, HERS, and IECC analysis.
1 Nominally averages 0.5 W/m2 of glazing (Sbar 2009). 2 The BEopt software tool was developed at NREL to identify optimal building energy designs aimed at minimizing the total of the amortized cost of improvements and the cost of energy. It produces designs that minimize combined construction and energy costs by using the DOE-2.2 and TRNSYS energy simulation programs to automate a sequential search technique for locating least-cost solutions on a path toward net zero energy. The software and underlying methodology are described in detail by Christensen et al. (2005, 2006) and Horowitz et al. (2008).
2
Atlanta was selected as an appropriate climate for assessing the energy-savings potential of EC windows, as it produces fairly balanced and substantial heating and cooling loads in residential buildings. Solar gain is beneficial at times because it reduces heating loads in the winter and undesirable at times as it contributes to cooling loads in the summer.
Two prototypical models were developed with the general characteristics outlined in Table 1, but differing performance characteristics. The first meets the requirements of the 2006 International Energy Conservation Code (IECC) for Atlanta, climate zone 3. The second includes performance characteristics of an Atlanta home using 50% less source energy than the Building America Benchmark, as described by Anderson and Roberts (2008). The performance characteristics of both are shown in Table 2.
Table 2. Prototype Model Performance Levels
Component 2006 IECC 50% Building America Walls 2 × 4, R-13 cavity 2 × 6, R-21 cavity Ceiling R-30 R-40 Infiltration 0.00050 SLA 0.00015 SLA Window U-value 0.65 0.30 Window SHGC 0.40 0.26 Lighting 14% fluorescent 90% fluorescent Air conditioner SEER 13 SEER 15 Furnace 80 AFUE 92 AFUE Ducts R-8, in crawl space In conditioned space Water heater 0.59 EF 0.77 EF Appliances Standard ENERGY STAR®
AFUE = Annual Fuel Utilization Efficiency EF = Energy Factor SEER = Seasonal Energy Efficiency Ratio SHGC = Solar Heat Gain Coefficient SLA = Specific Leakage Area
DOE-2 Modeling Capabilities The BEopt software was modified to accommodate and model EC windows in the DOE-2.2 simulation engine. DOE-2.2 provides a broad range of EC windows and controls. An EC window that best matches those currently available3
Table 3 was selected from the DOE-2.2
window library (see ).
3 Sage Electrochromics, Inc. is currently the only producer of commercially available EC glazing.
3
Table 3. Characteristics of SageGlass EC Glazing and DOE-2.2 Window Library Entry
U-Value SHGC Sage* DOE-2** Sage* DOE-2** Clear state 0.29 0.29 0.48 0.51 Tinted state 0.29 0.29 0.09 0.13
* SageGlass Classic™ (source: Sage Electrochromics Web site: www.sage-ec.com/pages/technol.html) ** DOE-2 2 library entries 2842/2843.
EC window controls available in DOE-2.2 include:
• Direct or total solar incident on the window • Direct or total solar transmitted through the glazing • Total horizontal solar • Outside temperature • Space load • Interior daylight level.
These controls can also be combined with a seasonal/hourly time schedule. We selected total solar incident on the window for control, as it allows the windows on each orientation to respond independently and is straightforward to implement in the field. DOE-2.2 provides for low and high limits for the control variable. We assumed that tinting begins when solar incident on the window exceeds 100 Btu/h·ft2; the window is fully tinted at 150 Btu/h·ft2. Figure 1 shows how the window tinting responds to incident solar in the DOE-2.2 model.
0
0.2
0.4
0.6
0.8
1
1.2
0 50 100 150 200 250 300
Total Incident Solar (Btuh/sf)
Frac
tion
Switc
hed
(1 =
Ful
ly T
inte
d)
Figure 1. EC window tinting response to total solar incident on window
4
Impact of Seasonal Control As mentioned earlier, it is beneficial to maximize solar gain during the heating season. Figure 2 shows the monthly source energy use for space heating and cooling for the Atlanta prototype model with standard glazing. Ideally, the EC windows would not reduce solar gain during periods with large heating loads and small cooling loads. We “locked out” the EC window control during the heating season in the BEopt/DOE-2 model and examined the impact. Based on the heating and cooling loads in Figure 2, the EC control was fixed in the “Clear” mode from October 15 through April 15, and then switched into “Auto” mode during the balance of the year. In “Auto” mode the glazing responds to total solar incident on the window.
Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec0
5,000
10,000
15,000
20,000
25,000
30,000
Coo
ling
(E) (
Btu
/hr)
Cooling (E)Heating (G)
Figure 2. Heating and cooling source energy use in Atlanta reference home
The energy and cost impacts associated with locking out the EC windows during the heating season are shown in Table 4 and Table 5. It is clearly beneficial to lock out automated window tinting during the heating season. Based on these results, automated EC control is assumed to function from April 15 to October 15 only.
Table 4. Source Energy Impacts of Disabling EC Control During Heating Season (MMBtu/yr)
Heating* Cooling* Total Automated year-round 71.2 29.3 100.5 Automated April 15 to October 15 67.1 29.9 97.0 Absolute savings 4.2 –0.6 3.6 Percent savings 5.9% –2.1% 3.5%
* Includes fan energy
5
Table 5. Energy Cost Impacts of Disabling EC Control During Heating Season ($/yr)
Heating* Cooling* Total Automated year-round 1105 202 1307 Automated April 15 to October 15 1040 206 1246 Absolute savings 65 –4 60 Percent savings 5.9% –2.1% 4.6%
* Includes fan energy
Response to Controls Figure 3 shows the level of window tinting on the west-facing windows during each hour of the year. The windows do not tint during the lock-out period between April 15 and October 15.
0
0.2
0.4
0.6
0.8
1
1.2
0 1000 2000 3000 4000 5000 6000 7000 8000
Hour of the Year
Frac
tion
Switc
hed
(1 =
Ful
ly T
inte
d)
Figure 3. EC window tinting for each hour of the year—west-facing window
Figure 4 shows window tinting for all orientations by hour of the day. This graph demonstrates how the windows tint in response to the position of the sun throughout the day.
6
0
0.2
0.4
0.6
0.8
1
1.2
1 6 11 16 21
Hour of the Day
Frac
tion
Switc
hed
(1
= F
ully
Tin
ted)
Front/EastLeft/SouthRight/NorthBack/West
Figure 4. EC window tinting for each hour of the day
Figure 5 quantifies the level of tinting that occurs on each orientation using “fully tinted hours”—the sum of all tinting that occurs throughout the year. As one might expect, with winter hours locked out, most tinting occurs in east- and west-facing windows. The north-facing windows never tint.
0
20
40
60
80
100
120
140
160
1 6 11 16 21
Hour of Day
Fully
Tin
ted
Equi
vale
nt H
ours
Front/East
Left/South
Right/North
Back/West
Figure 5. Fully tinted equivalent hours for each hour, by orientation
Results
Savings in a 2006 IECC-Compliant Home Results of BEopt runs compare the source energy of the 2006 IECC prototype home with EC windows to the home with code-level windows (see Figure 6). The EC windows result in house source energy. 9.1% savings in whole- Table 6 shows 18.2% source energy savings for the heating and cooling end uses; Table 7 shows analogous cost savings of 19.5%.
7
Misc. (G)
Hot Water (G)
Heating (G)
Cooling (E)
Cool Fan (E)
Heat Fan (E)
Lights (E)
Lg. Appl. (E)
Misc. (E)
Sou
rce
Ene
rgy
Use
(M
Btu
/yr)
36.3
28.3
32.2
28.4
77.9
19.9
36.3
28.3
32.2
25.7
61.5
19.9
0
50
100
150
200
250
Ref 1 - Design
236.6
215.0
Figure 6. BEopt output for source energy savings for EC windows relative to low-e, low-
SHGC windows
Table 6. Source Energy Savings From EC Windows in 2006 IECC home (MMBtu/yr)
Heating* Cooling* Total
2006 IECC-compliant home 85.2 33.3 118.5
2006 IECC with EC windows 67.1 29.9 97.0
Absolute savings 18.1 3.4 21.5
Percent savings 21.3% 10.2% 18.2%
* Includes fan energy
Table 7. Energy Cost Savings From EC Windows in 2006 IECC Home ($/yr)
Heating* Cooling* Total
2006 IECC-compliant home 1319 230 1549
2006 IECC with EC windows 1040 206 1246
Absolute savings 279 23 302
Percent savings 21.1% 10.2% 19.5%
* Includes fan energy
The BEopt/DOE-2 model predicts a peak-day electricity demand reduction of 0.7 kW, a 13.5% reduction in whole-house demand. Figure 7 shows the electricity demand for cooling for the two days with the highest predicted demand. Table 8 shows the demand savings at 7:00 p.m. on July 3, the peak hour for the year.
8
0
0.5
1
1.5
2
2.5
3
3.5
4
7/2 0:00 7/2 12:00 7/3 0:00 7/3 12:00 7/4 0:00
ECRef
Figure 7. Electricity demand for cooling with EC windows and code-compliant reference
windows in 2006 IECC home on July 2 and 3
Table 8. July 3 Peak Electricity Demand Savings From EC Windows in 2006 IECC-
Compliant Home (kW)
Cooling* Whole-House 2006 IECC-compliant home 3.7 5.5 2006 IECC with EC windows 3.0 4.7 Absolute savings 0.7 0.7 Percent savings 19.9% 13.5%
* Includes fan energy
Savings in a 50% Building America Home Results of BEopt runs comparing the source energy of the 50% Building America prototype home with EC windows, to the home with low-e, low-SHGC, argon-filled windows, are shown in Figure 8. The EC windows result in 3.2% savings in whole-house source energy. Table 9 shows 8.2% source energy savings for the heating and cooling end uses; Table 10 shows analogous cost savings of 9.4%.
9
Misc. (G)
Hot Water (G)
Heating (G)
Cooling (E)
Cool Fan (E)
Heat Fan (E)
Lights (E)
Lg. Appl. (E)
Misc. (E)
Sou
rce
Ene
rgy
Use
(M
Btu
/yr)
36.3
24.9
13.4
18.5
31.1
12.1
36.3
24.9
13.4
17.7
27.7
12.1
0
30
60
90
120
150
Ref 1 - Design
142.9138.3
Figure 8. BEopt output for source energy savings from EC windows in 50% Building
America home
Table 9. Source Energy Savings From EC Windows in 50% Building America Home (MMBtu/yr)
Heating* Cooling* Total
50% Building America home 33.6 21.1 54.7
50% Building America home with EC windows 30.0 20.2 50.2
Absolute savings 3.6 0.9 4.5
Percent savings 10.7% 4.2% 8.2%
* Includes fan energy
Table 10. Energy Cost Savings From EC Windows in 50% Building America Home ($/yr)
Heating* Cooling* Total
50% Building America home 524 145 670
50% Building America home with EC windows 468 139 607
Absolute savings 57 6 63
Percent savings 10.8% 4.2% 9.4%
* Includes fan energy
The BEopt/DOE-2 model predicts a peak-day electricity demand reduction of 0.4 kW, a 10.3% reduction in whole-house demand. Figure 9 shows the electricity demand for cooling for the two days with the highest predicted demand. Table 11 shows the demand savings at 7:00 p.m. on July 3, the peak hour for the year.
10
0
0.5
1
1.5
2
2.5
7/2 0:00 7/2 12:00 7/3 0:00 7/3 12:00 7/4 0:00
ECRef
Figure 9. Electricity demand for cooling (compressor and fan) with EC windows and
reference windows in 2006 IECC home on July 2 and 3
Table 11. July 3 Peak Electricity Demand Savings From EC Windows in 50% Building
America Home (kW)
Cooling* Whole House 50% Building America home 2.2 3.4 50% Building America home with EC windows 1.8 3.1 Absolute savings 0.4 0.4 Percent savings 16.4% 10.3%
* Includes fan energy
Cost-Competitive Electrochromic Windows We used the Atlanta 2006 IECC-compliant prototype model to run BEopt with a number of above-code features to identify the optimal non-EC windows for this home. BEopt identified low-e, low-SHGC, argon-filled windows as the most cost-effective upgrade at $16/ft2. This window was selected as an appropriate “competing technology” and the basis of this analysis. These same windows are in 50% Building America home.
To determine the cost at which EC windows will be competitive, we ran BEopt with EC windows at different price points: from $40/ft2 down to $10/ft2. Figure 10 shows the BEopt curves for these windows along with the “competing technology”: low-e, low-SHGC, argon-filled at $16/ft2. The graph indicates that EC windows would have to reach a price point of approximately $20/ft2 before they would be competitive with this window. EC windows currently cost $50–$100/ft2; residential applications are on the higher end (Sbar).
11
Mor
tgag
e +
Util
ities
($/
yr)
Source Energy Savings (%)
0
600
1200
1800
2400
3000
3600
4200
4800
5400
6000
0 10 20 30 40 50 60 70 80 90 100
Low-e low SHGC argElectrochromic $40Electrochromic $20Electrochromic $10Electrochromic $30
Low-e low SHGC argElectrochromic $40Electrochromic $20Electrochromic $10Electrochromic $30
$20/ft2 EC becomes
cost competitive
2006 IECC
Figure 10. BEopt output. EC glazing is competitive at approximately $20/ft2.
Conclusion A simple modeling study was undertaken to assess the potential energy savings of EC windows in residential applications. We used BEopt software to evaluate the performance of EC windows in prototypical energy models of a single-family home in Atlanta. The windows were assumed to operate automatically, tinting in response to incident solar during the cooling season. The model predicted the EC windows would produce whole-house source energy savings of 9.1%, and whole-house electricity demand savings of 13.5% in a 2006 IECC-compliant home and source energy savings of 3.2% and demand savings of 10.3% in a 50%-level Building America home. Comparing the cost and modeled performance of EC windows to “competing technology” indicates EC windows would be competitive at a price point of approximately $20/ft2.
Future Work Suggested future work to further assess the potential impact of EC windows in residential applications includes:
Additional modeling o Additional climates o Compare performance to orientation-tuned glazing o Examine other control strategies o Optimize controls
Monitoring installed performance o Test facility o Unoccupied lab homes o Occupied homes.
12
References Anderson, R.; Roberts, D. (2008). Maximizing Residential Energy Savings: Net Zero Energy Home (ZEH) Technology Pathways. Golden, CO: National Renewable Energy Laboratory, NREL/TP-550-44547. Christensen, C.; Horowitz, S.; Givler, T.; Courtney, A.; Barker, G. (2005). BEopt: Software for Identifying Optimal Building Designs on the Path to Zero Net Energy. Golden, CO: National Renewable Energy Laboratory, NREL/CP-550-37733. Christensen, C.; Anderson, R.; Horowitz, S.; Courtney, A.; Spencer, J. (2006). BEopt(TM) Software for Building Energy Optimization: Features and Capabilities. Golden, CO: National Renewable Energy Laboratory, NREL/TP-550-39929. Horowitz, S.; Christensen, C.; Brandemuehl, M.; Krarti, M. (2008). Enhanced Sequential Search Methodology for Identifying Cost-Optimal Building Pathways. Golden, CO: National Renewable Energy Laboratory, NREL/CP-550-43238. International Code Council, Inc. (2006). 2006 International Energy Conservation Code. ISBN-13: 978-1-58001-270-6. Lee, E.; Selkowitz, S; Clear, R; DiBartolomeo, D; Klems, J; Fernandes, L; Ward, G; Inkarojrit, V.; Yazdanian, M. (2006). Advancement of Electrochromic Windows, Sacramento, CA: California Energy Commission, PIER, 500-01-023, LBNL-59821. Sbar, N. (2009). Vice President Technology, Sage Electrochromics, Inc. Personal communication, October 23.
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Preliminary Assessment of the Energy-Saving Potential of Electrochromic Windows in Residential Buildings
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14. ABSTRACT (Maximum 200 Words) Electrochromic (EC) windows provide variable tinting that can help control glare and solar heat gain. We used BEopt software to evaluate the performance of EC windows in prototypical energy models of a single-family home in Atlanta. The windows were assumed to operate automatically, tinting in response to incident solar during the cooling season. The models predict the EC windows will produce whole-house source energy savings of 18.2% and whole-house electricity demand savings of 13.5% in a 2006 IECC-compliant home, and source energy savings of 8.2% and demand savings of 10.3% in a 50%-level Building America home.
15. SUBJECT TERMS electrochromic; ec; windows; tinting; glare; solar heat gain
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